Heat sensitive diazotype materials



United States Patent 3,431,109 HEAT SENSITIVE DIAZOTYPE MATERIALS Edward C. Bialczak, Mount Prospect, and William Murphy, Niles, Ill., assignors to Addressograph-Mulhgraph Corporation, Charles Bruning Division, Mount Prospect, Ill., a corporation of Delaware No Drawing. Filed Oct. 16, 1964, Ser. No. 404,484 US. Cl. 9649 4 Claims Int. Cl. G03c 1/60 ABSTRACT OF THE DISCLOSURE This invention concerns a heat developable diazotype reproduction material using a paper base precoated with a film of polyvinyl alcohol. A sensitizing solution containing a diazonium compound and an alkaline donor such as urea or thiourea is applied to the precoated surface of the paper base. The polyvinyl alcohol film prevents the alkaline donor from being absorbed by the paper base, thereby providing reproduction material having improved shelf life.

This invention relates to thermally responsive diazotype materials and more particularly to base coatings applied to the paper substrates prior to applying the light sensitive-thermal responsive coating which increase the shelf storage life of the finished material.

In the conventional and well-known diazotype reproduction processes the light sensitive diazotype material or copy sheet is exposed through a suitable original and then developed either by exposure to ammonia gas or by applying a developing solution directly to the exposed copy. The former is the dry or gas type development and the latter is known as the semi-moist process. The diazo coating in the exposed areas is photochemically decomposed and rendered unreactive. In those areas on the copy paper protected from light by the opaque portions of the original there remains the light sensitive diazo material which can be developed into a visible image by one of the aforementoned processes.

The light sensitive-heat responsive diazotype materials, with which this invention is concerned, are developed by exposure to heat. A typical thermally responsive diazotype material is prepared by coating a suitable raw stock material or substrate with a sensitizing solution comprising a light sensitive diazo component, an azo-coupling component, and an alkaline donor component. To make a copy, it is first exposed to actinic radiation in the same way as the conventional diazotype materials. The exposed material is then heated to a predetermined level which releases the active alkali from the alkaline donor producing a coupling reaction in the unexposed areas of the material.

The light sensitive-heat developed diazotype materials offer numerou advantages over the conventional am monia gas and the semi-moist type of development papers. The copying equipment is greatly simplified by virtue of the elimination of gas dispensing and developing chambers, as well as the undesirable presence of ammonia fumes. The operator need not bother with liquids as in the case of the semi-moist papers which are somewhat less convenient than simply heating the exposed copy paper to develop the azo-dye image.

One of the problems general to diazotype materials is their susceptibility to degradation on storage after the package is opened for use. This degradation manifests itself in the copy quality. The azo-dye images are of "ice low density, and the finished prints tend to have discolored backgrounds.

One of the serious deficiencies of the presently known thermally responsive diazotype materials which employ urea compounds as the alkali donor, is their low shelf life after the package is opened for use. This deficiency is one of the reasons why these materials have been unable to compete with the conventional type diazo materials. Under normal conditions, which, for the purpose of this invention is defined as room temperature and about 50% relative humidity, the image density decreases 45% to 55% from the images obtained from freshly coated material in a matter of a few weeks.

As the humidity of the storage environment increases the degradation is understandably accelerated. Under humidity conditions the materials when stored in an end-opened package have a useful life of only several days.

One of the causes of this rapid degradation is attributed to the fugitive nature of the alkaline donors employed. It is believed that the high water solubility of urea compounds renders them susceptible to migration or absorption into the paper substrate so that the donor concentration in the light sensitive-heat responsive coating diminishes.

The alkaline donors commonly used are urea compounds such as urea and thiourea. These materials are particularly suitable because of their tendency to break down at to 220 centigrade giving off ammoniacal fumes causing the aZo-coupling reaction to take place producing the azo-dye image. One of the notable characteristics of urea compounds is their high solubility in water which permits including sufficient amounts of the donor in the solution to produce the proper alkalinity.

It is a general object of the present invention to provide a thermally responsive diazotype material having an improved shelf life. It is a specific object of the present invention to provide a thermally responsive diazotype material having improved shelf life that is simple and inexpensive to manufacture. Other objects, features and advantages of the invention will become apparent on reference to the specification described hereinafter.

In accordance with the present invention the raw stock or substrate is coated with an aqueous solution of polyvinyl alcohol as a base coat over which is applied the light sensitive-heat responsive coating.

The use of polyvinyl alcohol compounds has been suggested for use in paper coatings where improved resistance to water penetration is required such as, for example, in the lithographic art.

The precise mechanism by which the polyvinyl alcohol functions when applied to paper substrate in the manner described is not known. It has been found that the objectives of this invention are best realized when the quantity of coating applied covers the loose, wild fiber formation found on the raw paper surface and yet does not form a distinct separate layer or continuous film since light sensitive-heat responsive coating must be absorbed into the substrate in order to yield a uniform coverage of the paper surface. As the coating weight goes below 0.5 gram per square meter the elfect of the coating is greatly diminished and above 3.0 grams per square meter it will tend to form a separate and distinct layer which would cause the sheet to lose flexibility, and might interfere with the application of light sensitive-heat responsive coating.

In the instant invention it has been found that polyvinyl alcohol provides a base upon which the light sensitive-heat responsive coating may be applied without fear of migration of the alkaline donor compounds into the substrate which is usually paper.

Papers prepared without the benefit of polyvinyl alcohol stored in opened packages show as much as a 45% loss in image density compared to only a loss using diazo materials embodying this invention.

The base treatment comprises a polyvinyl alcohol coating ranging from 0.5 gram to 3.0 grams per square meter of substrate applied from aqueous solution of from 2% to 10% by weight of polyvinyl alcohol, the preferred coating weight being 1.0 gram per square meter. It has been found that the grade of polyvinyl alcohol best suited for this application should contain less than 5% residual acetate. Material falling within this range is identified as being completely hydrolyzed polyvinyl alcohol. While the completely hydrolyzed grade is preferred, polyvinyl alcohols with greater amounts of residual acetate not exceeding to however, can be used with some measure of success, exhibiting not as good coverage properties as the preferred grades.

The aqueous polyvinyl alcohol solution is simple to prepare and may be applied from aqueous solutions using conventional roller coating in conjunction with air knife techniques.

Over the dried base the light sensitive-heat responsive coating is applied. In practice this is done on the same coating machine, being immediately applied over the base coat.

As discussed earlier, this invention finds particular utility in thermal systems employing urea as the heat responsive alkaline donor. Along with the alkaline donor are included a light sensitive diazo compound, couplers, and other constituents necessary to complete the coating to be discussed in detail hereinafter.

A number of light-sensitive diazo compounds can be used in preparing the light sensitive-heat responsive coating. Examples of such diazo compounds are the diazonium salts obtained by the diazotization of the following amines:

p-amino-N,N-diethylaniline p-amino-N-ethylaniline p-amino-N-ethyl-N-hydroxyethylaniline p-amino-N-methyl-N-hydroxyethylaniline p-amino-N,N-di-fl-hydroxyethylaniline p-amino-m-ethoxy-N,N-diethylaniline p-amino-N-ethyl-o-toluidine p-amino-N-ethyl-m-toluidine p-amino-N,N-diethyl-m-toluidine p-amino-N-ethyl-N-fi-hydroxyethyl-m-toluidine N-p-aminophenylmorpholine 1-amino-2,5-diethoxy-4-morpholinobenzene The preferred compound is the diazonium salt obtained from N-p-aminophenylmorpholine.

The coupling component may be selected from a large group of known compounds such as resorcinol, acetoacetanilide, 4-chlororesorcinol, diacetoacetylethylenediamine, phloroglucinol 1 phenyl-3-methyl-pyrazolone-(,5) and 2,3-naphthalenediol. The preferred coupler is 2,3-dihydroxynaphthalene-6-sulfonic acid. It has been shown that precoupling is a problem with many couplers; that is, color formation takes place before the material is ready for use.

Included in the coating formulation is the alkaline donor which is the component critical to the development of the dye image when exposed to heat. Exposure to heat causes the alkaline donor to decompose, releasing ammonia enabling the coupling reaction to proceed. Suitable alkaline donor compounds are, for example, urea and thiourea. These compounds are used to advantage because of their high solubility in water and their relatively low decomposition temperature. Accordingly, the light sensitive-heat responsive coating can tolerate large concentrations of urea and/ or thiourea, which will decompose at a temperature of about 150 C., releasing sufficient quantities of alkaline fume to produce the coupling reaction. Notwithstanding these desirable characteristics they tend to be fugitive under certain conditions of storage.

In the presence of the alkaline donors, the diazo salts and the couplers would, in and of themselves, tend to give a color reaction prematurely. To prevent precoupling and premature dye formation, there is added an acid component to keep the pH below the level required for coupling. Sulphonic acid derivatives, such as meta-benzenedisulphonic acid, are employed as the preferred acid component. Other acids may be used to produce the required pH, such as: maleic acid, malonic acid, ethanesulfonic acid, benzenesulfonic acid, oxalic acid, citric acid, tartaric acid, lactic acid.

In addition to the acid components, there may be added heavy metal salts such as the sulphates of zinc and cadrnium. The inclusion of the heavy metal salts tends to prevent precoupling of the components at near room temperature.

The following examples illustrate the present invention.

EXAMPLE 1 A base coating solution was prepared by mixing 3 grams of polyvinyl alcohol, sold by Shawinigan Resins Corporation under their trade name Gelvatol, having a maximum of 3% residual polyvinyl acetate, with 97 grams of water. To facilitate solution, the mixture is heated to about 80 C. under the influence of mechanical agitation. The cooled solution is applied to a 20 lb. substance paper substrate b roller coating or other suitable applicator means and forced air dried ready for the application of the light sensitive-heat responsive coating. A satisfactory base coat results from applying on a dry basis weight of from 0.5 gram to 3.0 grams of polyvinyl alcohol per square meter. In the instant example 1.0 gram of polyvinyl alcohol per square meter of dried paper was coated upon the substrate from the 3% solution.

The light sensitive-heat responsive coating was prepared by adding to mls. of water the following materials:

Urea gm 18 rn-Benzenedisulfonic acid gm 3 2,3-dihydroxynaphthalene-6-sulfonic acid gm 5 4-morpholinobenzenediazoniumchlorozincate -gm-.. 3 Zinc sulfate "gm..- 10 Rice starch gm 3 Polyvinyl acetate binder ml 8 Water to make 100 ml.

The materials were mixed at room temperature until the alkaline donor, diazo, acid, and coupler were all in solution and the mixture was then applied over the base coat at a loading of 15 mls. per square meter.

To prepare a reproduction of a translucent graphic original, it was placed on top of the diazotype material and exposed to actinic radiation. The exposed diazotype material was then passed between a pair of heated rollers having a surface temperature in the range of 150 C. to 220 C., thereby releasing the ammonia fumes from the alkaline donor causing the azo-dye formation. In the instant example, the 2,3-dihydroxynaphthalene 6-sulfonic acid produced a blue azo-dye image when coupled with the 4-morpholinobenzenediazoniumchlorozincate.

The density of the dye image prepared on the freshly coated paper read about 1.35 units measured on a standard densitometer. After one month of aging at room temperature and at about 50% humidity in an unsealed package, the image was to of its original density and after 3-5 months, the density was 70% to 75% of the density obtained with fresh paper.

EXAMPLE 2 A base solution comprising 10 grams of polyvinyl alcohol of the same grade as used in Example 1 was added to grams of water and agitated under heat at 80 C. until a particle free solution was obtained. The solution when cooled to room temperature had a viscosity of about 30 centipoises and was applied at a rate of 3.0 grams of polyvinyl alcohol per square meter of paper. This produced a somewhat thicker base coat when applied with standard roller coating equipment than that produced in Example 1.

The light sensitive-heat responsive coating applied over the base was the same as in Example 1. The same aging characteristics as obtained from the diazotype material of Example 1 were realized with the paper prepared in accordance with this example.

'. EXAMPLE 3 The base coat applied in this example is the same as in Example 1.

The light sensitive-heat responsive coating applied thereover was prepared by adding to 75 mls. of water the following materials:

Urea gm 12.5 Thiourea gm 12.5 Citric acid gm 1.5 Malonic acid gm 1.5 2,3-dihydroxynaphthalene-6-sulfonic acid gm 2.5 4-morpholinebenzene-diazoniumchlorozincate gm 2.0 Zinc sulfate gm 5.0 Rice starch gm 3.0 Polyvinyl acetate binder ml 8.0

Water to make 100.0 ml.

The alkaline donors, acid constituents, diazo salts and coupler were stirred until in solution and combined with the other ingredients until a uniform mixture resulted. The mixture was applied at a rate of mls. solution per square meter of paper to which the base coat had been applied.

In the instant example the alkaline donors are combined each having the desirable properties of high solubility and suitable temperature response.

Copies were produced from the fresh material in accordance with the general procedure described in Example 1. The paper had image densities in the range of 1.30-1.40 units measured on a standard densitometer.

After storage in an end-opened package for 21 days under standard conditions, measurement of the image density obtained on the densitometer read 1.1 units. The image retained 85% of its original density.

EMMPLE 4 In the example the base coat as set forth in Example 2 was applied to a paper substrate over which was coated the following light sensitive-heat responsive coating. To 75 mls. of water were added:

Urea gm 25.0 Citric acid gm 1.5 Malonic acid gm 1.5 Resorcinol gm 2.0 2,3-hydroxynaphthalene-6-sulfonic acid gm 5.0 4-morpholinebenzene-diazoniumchlorozincate gm 2.0 Zinc sulfate gm 15.0 Rice starch gm 3.0 Polyvinyl acetate binder ml 3.0

Water to make 100.0 ml.

Mixing and coating procedures were the same as in Example 1 as was the copying procedure to make a print.

This material produced a blackline print upon development with heat. The freshly coated paper yielded image densities in the range of 1.3-1.4 units measured on a densitometer.

The aging characteristics were excellent. Only slight degradation occurred in open package storage for 1 month during standard conditions producing azo-dye images measuring 1.1 units on a standard densitometer.

EXAMPLE 5 A base coat prepared with a grade of polyvinyl alcohol not completely hydrolyzed gave some measure of protection against the fugitive character of the urea and thiourea alkaline donor components. The material, Gelvatol, manufactured by Shawinigan Resins, under its grade designation 20-30 having less than 22% residual acetate was formulated as follows:

To mls. of water was added 10 grams of 20-30 grade polyvinyl alcohol and to aid solution the mixture was heated to about 90 C. With increased acetate con tent the grade of polyvinyl alcohol enjoys less water solubility. When solution is complete the formulation is applied to 20 lb. substance paper raw stock at a rate which yields a dried coating of 1.0 gram per square meter.

Over the base coat may be applied any one of the light sensitive-heat responsive coatings described in Examples 1, 3 and 4. The aging performance of the papers was improved over nonprecoated paper but slightly less effective than the results obtained with the completely hydrolyzed grade.

We claim:

1. The method of making a contact copy of an original on a diazotype sheet comprising the steps of exposing imagewise said copy sheet to actinic radiation, said sheet comprising a paper substrate comprising a light-sensitive, heat-responsive coating applied to one surface thereof consisting essentially of a light-sensitive diazonium compound, an azo coupling component and an alkaline donor component selected from the group consisting of urea and thiourea in the ratio of 6.02125 parts by weight of said alkaline donor per part by weight of said light-sensitive component, said surface further characterized in having a barrier-type base coat consisting essentially of polyvinyl alcohol not having more than 20%-25% residual acetate applied thereto at the rate of from 0.5 gram to 3.0 grams of polyvinyl alcohol per square meter on a dry basis prior to the application of said light-sensitive, heat-responsive coating thereon, and developing a visible azo dye image on said copy sheet by heating said sheet to about C.

2. A heat developable diazotype reproduction material comprising a paper substrate having applied thereto a discontinuous barrier-type film coating consisting essentially of polyvinyl alcohol not having more than 20%- 25% residual acetate at a rate of from 0.5 gram to 3.0 grams of polyvinyl alcohol per square meter on a dry basis and a diazotype coating applied over said discontinuous barrier-type film coating comprising a heat-responsive, light-sensitive diazonium component, an azo coupling component and an alkaline donor component selected from the group consisting of urea and thiourea, said alkaline donor component being present in the range of 6 parts to 12.5 parts by weight per part by weight of light-sensitive diazonium component whereby said alkaline donor component is prevented from migrating into said paper base support.

3. The diazotype reproduction material of claim 1 wherein said polyvinyl alcohol contains less than 5% of residual acetate groups.

4. The diazotype reproduction material of claim 1 wherein an azo dye image is produced when exposed to heat at about 150 C.

References Cited UNITED STATES PATENTS (Other references on following page) 3,431,109 7 8 FOREIGN PATENTS pers, Photo Science & Eng., v01. 5, No. 4, July-August 713,917 8/1954 Great Britain. 196109239443- 749,034 5/1956 Great Britain.

4 1 7/1960 Great i i J. TRAVIS BROWN, Primary Examiner. 865,680 4/1961 Great Britain. 5

909,491 10/1962 Great Britain C. BOWERS, Asszstant Exammer. 637,448 2/ 1967 Canada. U.S. Cl. X.R.

OTHER REFERENCES 9645 Kosar, J.: Thermal Development of Diazotype Pa- 

